Lead metal (Pb) and its compounds are well known toxins. Despite this, lead is a component of solder alloys in widespread use in the electronic industry. For example, a binary eutectic Sn-Pb (e.g. 63 weight % Sn-37 weight % Pb--melting point of 183.degree. C.) solder and Pb-5 weight % Sn solder are in widespread use for electronic and circuit joining applications. However, in the United States, there is proposed Congressional legislation to ban, or possibly heavily tax, the use of lead in all commercial products, such as, for example, electronic solders.
In response to the possibility of a governmental ban or excessive taxation of lead use, manufacturers and users of Pb-bearing solder for electronic and circuit joining applications recently have attempted to developed Pb-free solders especially as direct replacements for the conventional Sn-Pb solders in conventional solder reflow procedures employing general heating of an electronic wiring board to temperatures of 230.degree. to 250.degree. C.
In particular, binary eutectic Sn-Ag, Sn-Sb and Sn-Bi solders have been developed as Pb-free solders. For example, a 96.5 weight % Sn-3.5 weight % Ag solder, 95 weight % Sn-5 weight % Sb and 43 weight % Sn-57 weight % Bi solder have been developed. These Pb-free solders exhibit eutectic melting points of 221.degree. C., 245.degree. C. and 139.degree. C., respectively. These solders are described by Sungho Jin in a series of articles in Journal of Metals, July, 1993; namely, "Developing Lead-Free Solders: A Challenge and Opportunity," JOM, 45, no. 7, page 13, (1993). The Sn-Ag solder melting point is described as "slightly too high", while the Sn-Sb solder is described as having a "melting point too high". The Sn-Bi solder melts at a low temperature but suffers from rapid microstructural coarsening, poor wetting, and poor creep resistance.
Other Pb-free binary eutectic solders comprising Sn-In and Sn-An have been developed. However, these binary solders are disadvantageous in that both are brittle and include relatively large amounts; e.g. 50.9 weight % of In or 80 weight % of Au that are costly alloy components and not as readily available as previously used alloy components. Also, the Sn-Au solder has an elevated eutectic temperature of 278.degree. C. that is too high for conventional solder reflow temperatures.
The aforementioned Pb-free binary solders have been used as replacements for the conventional Sn-Pb solders in some solder fellow procedures employing general heating of an electronic wiring board to non-standard temperatures.
A ternary, off-eutectic Sn-Cu-Ag solder has been developed by the plumbing industry as a Pb-free solder with, for example, 96 weight % Sn-3.5 weight % Cu-0.5 weight % Ag. This plumbing solder has a 227.degree. C. solidus temperature and 260.degree. C. liquidus temperature which would be considered too high by electronics manufacturers. Moreover, this solder exhibits a "mushy" solid-liquid zone of 33.degree. C. that is too great for electronic soldering.
As microelectronic technology continues to evolve, microprocessors are becoming more complex and in all likelihood will generate more heat and increase thermal strains in electronic and circuit solder joints. Moreover, the service environment of electronic solder joints in all probability will become more severe. For example, electronic packages are now required to be positioned closer to the engine in automobiles for faster response and greater cost effectiveness. To produce automobiles with increased fuel efficiency, lower hoods and smaller front mounted air inlets are provided to streamline the automobile body. These changes subject automotive electronics packages in the engine compartment to higher operating temperatures and higher thermal strains. In emerging avionics applications, electronic packages used for sensing, control, and telemetry are exposed to severe ambient temperature cycles that create cyclic thermal strains at solder joints of the electronic packages used.
As a result of the more severe service demands being imposed on electronic packages in the automotive, avionics and other fields, there is a need for a solder that not only is Pb-free and amenable to the aforementioned conventional solder fellow procedures but also is stronger and more fatigue resistant than the Pb-free binary alloy solders (e.g. Sn-Bi and Sn-In) discussed hereabove. Moreover, there is a need for such a solder that would be competitive in cost and as readily available as conventional Sn-Pb eutectic solders for high volume use.